In the field of digital radio beams, modems are using modulation of ever increasing density. Only coherent demodulation is then possible since differential demodulation would give rise to too great a threshold displacement when using modulation having a very large number of states. Such coherent demodulation is based on the reliability and the quality of the carrier phase estimator as generated at the reception end.
Beyond 4-state phase modulation (4-PSK type) sampled estimators are used. They rely on samples of the demodulated signals being available, which samples are taken at the symbol rate and at the instant of maximum opening in the eye diagram. These samples come from analog-to-digital converters and they enable the transmitted symbols to be estimated and also the associated distortion to be evaluated.
For technological reasons, the conversion-estimation grid has been solved up to now by being reduced to useful information bits defining the symbol plus one or two extra bits giving the distortion.
The article by A. Leclert and P. Vandamme entitled "Universal carrier recovery loop for QASK and PSK signal sets" (Globecom '82 Conf. Rec., Vol. 3, pp. 1228-1232, Nov. 29-Dec. 2, 1982, Miami, FL) describes a carrier loop applicable to quadrature amplitude shift keying (QASK) and to phase shift keying (PSK). This article presents several QASK-16 estimation and conversion systems.
Such regeneration and estimation structures are now conventional and simplified gradient algorithms have been constructed for them based on knowing the sign of the received signal and the sign of the corresponding distortion. This family of estimators directs control of the regenerator (carrier, sync., amplitude) and of the time equalizer. The carrier estimator relies on calculating the effect of the in-phase X channel on the quadrature Y channel (sgnX.sgn.sub.err Y) and the effect of the Y channel on the X channel (sgnY.sgn.sub.err X).
However such a conventional estimator suffers from the drawback of seeing its monotonic range reduced to the range for which the distortion estimate is valid. This means that the criterion is valid only if the demodulated constellation has meaningful correspondence with the estimation grid. This condition restricts the possibilities of achieving phase locking to low levels of noise and of distortion.
An object of the present invention is to solve this problem by proposing a robust carrier recovery criterion applicable to modulation using a large number of states and capable of accommodating high levels of noise or of distortion.